U.S. patent number 4,911,761 [Application Number 07/184,544] was granted by the patent office on 1990-03-27 for process and apparatus for drying surfaces.
This patent grant is currently assigned to CFM Technologies Research Associates. Invention is credited to Christopher F. McConnell, Alan E. Walter.
United States Patent |
4,911,761 |
McConnell , et al. |
March 27, 1990 |
Process and apparatus for drying surfaces
Abstract
Object surfaces such as semiconductor wafers which are suspended
in a rinsing fluid may be dried by replacing the rinsing fluid,
such as water, with a drying vapor by directly displacing the water
from the surfaces at such a rate that substantially no liquid
droplets are left on the surfaces after replacement of the water
with drying vapor. Preferably, the drying vapor is miscible with
water and forms a minimum-boiling azeotrope with water, such as
isopropanol. The drying vapor is then purged with a stream of dry,
inert, non-condensable gas such as nitrogen. A vaporizer with
automatic refill mechanism produces saturated drying vapor which
may then be flashed to a superheated vapor prior to contacting the
surfaces, which preferably are at the same temperature as the
vapor. Preferably, no liquid is removed by evaporation, and the
drying takes place in an enclosed, hydraulically full system which
does not require movement or handling of the surfaces between
rinsing and drying steps.
Inventors: |
McConnell; Christopher F. (West
Chester, PA), Walter; Alan E. (Exton, PA) |
Assignee: |
CFM Technologies Research
Associates (Lionville, PA)
|
Family
ID: |
40800439 |
Appl.
No.: |
07/184,544 |
Filed: |
April 20, 1988 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
765294 |
Aug 13, 1985 |
4778532 |
|
|
|
747894 |
Jun 24, 1985 |
4633893 |
|
|
|
612355 |
May 21, 1984 |
4577650 |
|
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Current U.S.
Class: |
134/11; 134/10;
134/30; 134/31 |
Current CPC
Class: |
B05C
3/109 (20130101); B08B 3/04 (20130101); B08B
7/0092 (20130101); H01L 21/67028 (20130101); B08B
2203/002 (20130101); Y10S 134/902 (20130101) |
Current International
Class: |
B05C
3/09 (20060101); B05C 3/109 (20060101); B08B
7/00 (20060101); B08B 3/04 (20060101); H01L
21/00 (20060101); B08B 003/08 () |
Field of
Search: |
;134/10,11,30,31 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Cleaning Techniques for Wafer Surfaces," Semiconductor
International, Aug. 1987, pp. 80-85..
|
Primary Examiner: Pal; Asok
Attorney, Agent or Firm: Panitch Schwarze Jacobs &
Nadel
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of our copending
application Ser. No. 765,294, filed Aug. 13, 1985 for "Process and
Apparatus for Treating Wafers with Process Fluids", now U.S. Pat.
No. 4,778,532, which is a continuation-in-part of copending
application Ser. No. 747,894, filed June 24, 1985 for "Apparatus
for Treating Semiconductor Wafers," now U.S. Pat. No. 4,633,893,
which is a continuation-in-part of copending application Ser. No.
612,355, filed May 21, 1984, for "Vessel and System for Treating
Wafers with Fluids," now U.S. Pat. No. 4,577,650, and is related to
copending application Ser. No. 747,895, filed June 24, 1985 for
"Vessel and System for Treating Wafers with Fluids." The disclosure
of each of these prior applications is incorporated herein by
reference.
Claims
We claim:
1. A method for drying surfaces of objects which are suspended in a
rinsing fluid comprising providing a drying vapor, replacing said
rinsing fluid with said drying vapor by directly displacing said
rinsing fluid from said surfaces with said vapor at such a rate
that substantially no liquid droplets are left on the surfaces
after replacement of the rinsing fluid with drying vapor.
2. A method according to claim 1 wherein said rinsing fluid is
water in the liquid phase.
3. A method according to claim 2 wherein said surfaces are heated
to approximately the temperature of said drying vapor prior to
being contacted by said vapor.
4. A method according to claim 3 wherein said heating is effected
by said rinsing fluid as the heat transfer means.
5. A method according to claim 3 wherein said heating is effected
by solid/solid heat transfer from a carrier which suspends said
objects.
6. A method according to claim 1 wherein said rinsing fluid is
pushed downwardly by said drying vapor.
7. A method according to claim 1 wherein said rinsing fluid is
drawn away from said objects by external pumping means.
8. A method according to claim 1 wherein said drying vapor is
purged from said surfaces by introducing a dry, inert,
non-condensable gas after replacement of said rinsing fluid.
9. A method according to claim 8 wherein said gas is nitrogen.
10. A method according to claim 1 wherein said drying vapor is
saturated.
11. A method according to claim 1 wherein said drying vapor is
superheated.
12. A method according to claim 2 wherein said vapor is miscible
with water.
13. A method according to claim 2 wherein said vapor forms a
minimum-boiling azeotrope with water.
14. A method according to claim 13 wherein said vapor is
isopropanol.
15. A method according to claim 1 wherein said vapor is an
azeotrope.
16. A method according to claim 2 wherein said drying vapor is an
isopropanol/water azeotrope.
17. A method according to claim 1 wherein substantially no rinsing
fluid or drying vapor is removed by evaporation of liquid
droplets.
18. A method according to claim 1 wherein said drying vapor is an
organic compound non-reactive with said surfaces and having a
boiling point less than 140 degrees Centrigrade at atmospheric
pressure.
19. A method according to claim 1 which does not require movement
or handling of said surfaces between rinsing and drying steps.
20. A method according to claim 19 wherein the vessel in which said
objects are suspended is hydraulically full during said rinsing and
drying steps.
21. A method according to claim 20 wherein said objects are
blanketed with drying vapor immediately after removal of the
rinsing fluid.
22. A method according to claim 1 wherein said objects are
semiconductor wafers.
23. A method according to claim 1 wherein said vapor is filtered in
the vapor phase prior to contacting said surfaces.
24. A method according to claim 1 wherein said drying vapor is
collected and recycled after drying said surfaces.
25. A method according to claim 24 wherein said vapor is recycled
in the form of an azeotrope with said rinsing fluid.
26. An enclosed, full flow method for drying semiconductor wafers
which are immersed in a vessel containing a hot water rinsing
liquid comprising introducing isopropanol vapor from above the
wafers to remove and replace said water as the water level recedes
downwardly from the wafers at such a rate that substantially no
liquid droplets are left on the wafer surfaces, said wafers being
at substantially the same temperature as said vapor when contacted
by the vapor.
27. An enclosed, full flow method for drying surfaces of objects
which are immersed in a vessel containing a rinsing fluid
comprising introducing a drying vapor from above the objects to
remove and replace the fluid, such that the fluid level recedes
downwardly from the objects at such a rate that substantially no
liquid droplets are left on the object surfaces after replacement
of the rinsing fluid with the drying vapor.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to the drying of surfaces after
fluid treatment or wet processing. More particularly, the invention
relates to the manufacture of semiconductor components, and in
particular to the preparation of semiconductor wafers prior to high
temperature processing steps, such as diffusion, ion implantation,
epitaxial growth, and chemical vapor deposition steps. Still more
particularly, the invention relates to methods and apparatus for
the drying of semiconductor wafers after prediffusion cleaning.
2. Prior Art
In the fabrication of semiconductor wafers several process steps
require contacting the wafers with fluids. Examples of such process
steps include etching, photoresist stripping, and prediffusion
cleaning. The equipment conventionally used for contacting
semiconductor wafers generally consists of a series of tanks or
sinks into which racks of semiconductor wafers are dipped. Wafer
carriers are described, for example, in U.S. Pat. Nos. 3,607,478,
3,964,957 and 3,977,926. Such conventional wet processing apparatus
poses several difficulties.
Since the tanks are open to the atmosphere, airborne particulates
can enter into the process solutions. Through surface tension these
particles are easily transferred to the wafer surfaces as the
wafers are dipped into and lifted out of the sinks. This
particulate contamination is extremely detrimental to the
microscopic circuits which the wafer fabrication process creates.
It is especially important to minimize particular contamination
during prediffusion cleaning.
After fluid processing the wafers normally need to be dried. This
can be a particularly challenging process because it is important
that no contamination be created during the drying process.
Evaporation is undesirable since it often leads to spotting or
streaking. Even the evaporation of ultra high purity water can lead
to problems because such water is very aggressive to the water
surface and will dissolve traces of silicon and silicon dioxide
during even short periods of water contact. Subsequent evaporation
will leave residues of the solute material on the wafer surface.
Contamination and other causes of semiconductor failure are
discussed, for example, in J. Schadel, "Device Failure Mechanisms
In Integrated Circuits," Solid State Devices 1983 Conf. Ser. No. 69
(Institute of Physics, London 1984) 105-120.
Conventionally, semiconductors are dried through centrifugal force
in a spin-rinser-drier. Because these devices rely on centrifugal
force to "throw" water off the wafer surfaces, their use results in
several problems. First, there are mechanical stresses placed on
the wafers which may result in wafer breakage, particularly with
larger wafer sizes. Second, because there are many moving parts
inside a spin-rinser-drier, contamination control becomes a
difficult problem. Third, since the wafers conventionally travel at
high velocity through dry nitrogen, static electric charges develop
on the wafer surfaces. Since oppositely charged airborne particles
are quickly drawn to the wafer surfaces when the spin-rinser-drier
is opened, particular contamination results. Fourth, it is
difficult to avoid evaporation of water from the surfaces of the
wafers during the spin process with the attendant disadvantages
discussed above.
More recently, methods and apparatus have been developed for steam
or chemical drying of wafers, including the method and apparatus
disclosed in our U.S. Pat. No. 4,778,532. Chemical drying generally
comprises two steps. First, the rinsing fluid, preferably water is
driven off the wafers and replaced by a nonaqueous drying fluid.
Second, the nonaqueous drying fluid is evaporated using a predried
gas, preferably an inert gas such as nitrogen at a low flow
velocity.
Another chemical drying process currently used in Japan consists of
sequentially immersing the wafer carrying vessel in tanks of
deionized water, followed by suspending the wafers above a tank of
boiling isopropanol. The wafer-carrying vessel is then slowly
withdrawn from the isopropanol vapor to pull the water droplets off
the wafer surfaces.
The most important feature for an effective wafer drying technology
is that the wafers produced by ultraclean, i.e., with minimum
particle contamination and minimum chemical residue. Because many
drying solvents are flammable, safety is also a very important
consideration. Other important design criteria include low chemical
consumption, low waste generation, and automated handling with
little or no operator exposure.
BRIEF SUMMARY OF THE INVENTION
According to the present invention, a method and apparatus are
provided for drying surfaces of objects such as semiconductor
wafers after rinsing the surfaces to remove process fluids such as
etchants, photoresist strippers or prediffusion cleaners. A drying
vapor is supplied to the surfaces in such a manner that the vapor
replaces the rinsing fluid by directly displacing the rinsing fluid
on the surfaces at such a rate that substantially no liquid
droplets are left on the surfaces after replacement of the rinsing
fluid with drying vapor. Preferably, the drying vapor is provided
from above the objects in a fully enclosed, hydraulically full
system, and the drying vapor pushes the rinsing liquid off the
surfaces as the liquid level recedes downwardly.
Since the rinsing fluid will usually be water in the liquid phase,
it is preferred that the drying vapor be miscible with water and
form a minimum boiling azeotrope with water, isopropanol being a
particularly preferred drying vapor. Further, the drying vapor
should be substantially pure and either saturated or preferably
superheated, and the surfaces should be heated to a temperature
near to but preferably below that of the drying vapor prior to
contact. While this may result in some condensation of vapor on the
surfaces, and too much condensation should be avoided, it has been
found that preheating to above the temperature of the drying vapor
yields poorer results.
The method and apparatus further include a supply of dry, inert,
non-condensable gas to purge the drying vapor after replacement of
the rinsing fluid, and a boiler and distillation column for
concentrating the mixture of rinsing fluid and drying vapor after
exit from the processing vessel.
In a preferred embodiment, a vaporizer for producing the saturated
drying vapor is provided with a lower boiler section and an upper
holding section in which the drying vapor may be rapidly produced
in the boiler section and held in the upper section at just the
boiling point of the drying fluid. This may be accomplished by an
insulating gasket to limit the amount of heat transfer from the
boiler section to the holding section. The vaporizer is preferably
totally enclosed and may be automatically replenished with fresh
drying fluid by lowering the temperature of the fluid in the upper
holding section to create a subatmospheric pressure which will draw
fresh drying fluid into the vaporizer from a storage source which
is preferably maintained below the temperature of the vaporizer.
Saturated drying vapor with an appropriate latent heat curve
(pressure-enthalpy diagram) is preferably superheated by means of a
pressure drop valve and filtered between the vaporizer and the
processing vessel.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing summary, as well as the following detailed
description, will be better understood when read in conjunction
with the appended drawings. For the purpose of illustrating the
invention, there is shown in the drawings an embodiment which is
presently preferred, it being understood, however, that this
invention is not limited to the precise arrangement and
instrumentality shown in the drawings.
FIG. 1 is a schematic flow diagram illustrating a preferred
embodiment of the apparatus of the present invention.
FIG. 2 is a slightly enlarged, more detailed, cross sectional view
of the wafer vessel shown in FIG. 1, and illustrating the
gas-liquid-solid interface in the wafer vessel.
FIG. 3 is a cross sectional view similar to FIG. 2 but illustrating
an alternate wafer vessel and carrier arrangement.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
While the present invention is directed broadly to the drying of
surfaces of solid objects, particularly planar objects, after
various wet processing or fluid treatments, the invention will be
described with particular reference to the drying of semiconductor
wafers after prediffusion cleaning and rinsing, it being understood
that the same general principles apply to the drying of other wet
surfaces.
Further, while the drying system of the present invention may be
used with a variety of cleaning, etching or other wafer processing
methods and apparatus, the present system is specifically adapted
for use in the process and apparatus for treating wafers with
process fluids, as described and claimed in our U.S. Pat. No.
4,778,532. Thus, the present invention is an improvement which may
be substituted for the steam or chemical drying systems described
in that application.
Similarly, while various wafer carrying vessels may be used to
suspend the semiconductor wafers in the rinsing and drying fluids
as described herein, the wafer carrier described in our U.S. Pat.
No. 4,577,650 has been found to be particularly suitable for use in
the method and apparatus of the present invention. Such carriers
are shown in simplified form in FIG. 2 of the present application
for purposes of illustrating the present invention.
The apparatus for carrying out the present invention includes three
primary pieces of equipment, namely a vaporizer 10 for producing
drying vapor, a vessel 12 for holding the wafers for treatment with
rinsing fluid and drying vapor, and a boiler 14 for concentrating
used rinsing fluid and drying vapor for disposal and/or reuse.
These three units with their associated piping, valves and other
elements are shown schematically in FIG. 1.
Referring first to FIG. 2, a preferred vessel 12 for use in the
present invention is illustrated. A plurality of semiconductor
wafers 16 is shown (from a side view) suspended in two rows of
parallel, vertically oriented wafers in upper and lower wafer
carriers 18, 20, of the type described more fully in our U.S. Pat.
No. 4,577,650. While two such wafer carriers 18, 20 are illustrated
in FIG. 2 stacked one on top of the other, it will be understood
that vessel 12 could include only one such wafer carrier or more
than two such carriers. In some instances vertical stacking may
cause undesirable dripping onto lower carriers.
Wafer carriers 18, 20 are held in place by upper vessel clamp 22
connected to the fluid inlet 24 and lower vessel clamp 26 connected
to fluid outlet 28. In FIG. 2 vessel 12 is shown partially full of
rinsing liquid 30 as drying vapor 32 fills the upper portion of
vessel 12 and pushes the gas-liquid-solid interface 34 downwardly
across wafers 16 in upper wafer carrier 18.
In FIG. 3 a vessel 12' is shown which allows for a plurality of
SEMI (Semiconductor Equipment and Materials Institute, Inc.)
approved wafer carriers to be arranged side by side. In this
embodiment upper vessel clamp 22' is in the form of a bell jar lid
which is sealed over lower vessel clamp 26' by means of a gasket 25
and clamp fittings 27. Wafer vessels 18', 20', holding wafers 16',
are supported on rods 21 in lower vessel clamp 26'. The vessel 12'
is shown empty of rinsing fluid.
It will be understood that other vessel arrangements (not shown)
can be used in carrying out the present invention. For example,
another embodiment would allow for one or more wafer carriers to be
placed in an overflow-type sink situated in a lower vessel clamp
with an appropriate cover, such as a bell jar lid similar to upper
vessel clamp 22' in FIG. 3, to seal the system and provide for
overflow and waste removal. Sealing the system is important to
allow blanketing the vapor space around the wafers with drying
vapor to prevent the introduction of foreign gases, e.g., nitrogen
gas, during the direct displacement of the rinsing fluid with the
drying fluid.
Referring to FIG. 1, vaporizer 10 includes a lower boiler section
36 and an upper holding section 38, with the metal casings of the
boiler and holding sections being separated by an insulating gasket
40 which limits the amount of heat transfer from the lower metal
casing to the upper metal casing.
Boiler 36 is provided with heating bands 42 or other suitable heat
transfer device to quickly heat the drying fluid above its boiling
point. The boiler 36 should always be maintained full of liquid
drying fluid so that the heat transfer surfaces are continually
immersed. For this purpose a liquid level detector and switch (not
shown) may be provided, as well as resistance temperature detectors
(not shown) for measuring the heat of the drying liquid and
monitoring the temperature of the heating bands.
The insulating gasket 40 may be of any suitable material which will
withstand the heat of boiler section 36 and will resist corrosion
by the drying fluid. For example, an envelope gasket which will fit
between two ANSI flanges 44 of the boiler section 36 and holding
section 38 will be suitable. The insulating gasket prevents
troublesome temperature/pressure overshoot from rapid heatup of the
metal casing without simultaneous heatup of the contained drying
fluid. Thus, while the boiler may be very hot to quickly produce
drying vapor, the holding section 38 should remain at just the
temperature of the boiling drying fluid, which may be an elevated
pressure boiling point.
The holding section 38 essentially holds the liquid capacity of the
vaporizer, with the disengagement between liquid and vapor
occurring at the very top, near flanges 46. A similar insulating
gasket 48 may be placed between the flanges 46 to avoid unnecessary
heat transfer from the top of holding section 38 near the liquid
vapor interface. Holding section 38 may be provided with other
associated devices (not shown), including a level detector and
switch, heat tracing, and water cool-down jacket.
Drying liquid and vapor enter and leave holding section 38 through
tubing 50 and cross connection 52. Fresh and/or recirculated liquid
drying fluid enters vaporizer 10 through valve 54, preferably a
bellows valve, and filter 56, preferably a submicron filter such as
available under the trademark Millipore, in inlet line 58 which is
connected to a source of fresh drying fluid (not shown) through 53
and/or to distillation column 94 and waste liquid receiver 95 which
provide recirculated drying fluid as discussed below.
Saturated drying vapor flows to vessel 12 through valve 60,
preferably also a bellows valve, and flash valve 62 in line 64. As
will be described more fully below, the flash valve may be use to
lower the pressure and thereby superheat the saturated drying
vapor. After passing through flash valve 62 and 3-way selector
valve 68, the drying vapor preferably also goes through a final
0.01 micron ceramic filter 69, such as a Model PGF-2 filter
manufactured by Fastek division of Eastman Technology, Inc.,
because gases can be filtered to a higher degree than liquids.
Cross connection 52 is also provided with a pressure release valve
66 for emergency over pressurization of the vaporizer. The
vaporizer may also be provided with a maintenance drain (not shown)
at the bottom of the boiler 36.
Vessel 12 may be provided with other valves 70, 72, 74 and 76 for
the control of various process fluids, such as etching, stripping,
cleaning and/or rinsing fluids which may enter and exit the vessel
12 for treatment of wafers 16. The manner in which these fluids may
be controlled for entry into and exit from vessel 12 is described
in more detail, for example, in our U.S. Pat. No. 4,778,532, but
does not form a part of the present invention.
As drying fluid displaces rinsing fluid from wafers 16 at interface
34, the drying fluid mixes with the rinsing fluid and also forms a
distinctive drying fluid layer on top of the rinsing fluid,
reaching more than one half inch in thickness in some cases.
This final rinsing fluid and drying fluid layer exit vessel 12
through valve 78 or metering pump 79 in line 80 which leads to
boiler 14 for concentration and/or disposal of the used fluids.
Preferably metering pump 79 is a variable rate pump to allow better
control of the interface descent rate and to optimize drying time.
Just preceding valve 78 and metering pump 79 in line 80 is a
capacitance switch (limit switch) 82 which senses when vessel 12
has drained completely. At that point vapor line 64 is closed and a
purging gas may be passed into vessel 12 through valves 71, 68 and
70 and filter 69.
Drying vapor and purging gas may also exit vessel 12 through valve
78 and pass into boiler 14. To achieve even better control of the
descent rate of interface 34 and to optimize drying time, the
rinsing liquid may be removed during at least part of the descent
by a variable rate metering pump 79. The rinsing fluid is discarded
to drain through valve 81. At the appropriate time, the layer of
drying fluid and a layer of rinsing fluid immediately below it are
diverted to boiler 14 through valve 83.
Boiler 14 is provided with band heaters 92 or immersion heaters to
strip the drying fluid or an azeotrope of the drying fluid and
rinsing fluid from the waste water. The vapor goes through
distillation column 94 for further concentration. A water-cooled
condenser 86 condenses the drying vapor. Cool, non-condensable gas
(e.g., purge gas) exits the condenser through vent 88, while a
portion of the condensate exits through drain 90 into a waste
liquid receiver 95 for recirculation to the feed line 58 for
vaporizer 10.
Distillation column 94 may be a single column or a series of
columns of generally conventional design, depending upon the degree
of concentration of the drying fluid desired for recirculation or
disposal of used fluid. The waste water from which the vapor has
been stripped exits boiler 14 through overflow valve 96 as a new
batch of used fluid enters boiler 14 from the next run. Fresh
drying fluid may be added to the recycled fluid through valve
83.
In carrying out the method of the present invention, a drying fluid
is selected which is miscible with the rinsing fluid and preferably
forms a minimum-boiling azeotrope with the rinsing fluid. Since
water is the most convenient and commonly used rinsing fluid, a
drying fluid which forms a minimum-boiling azeotrope with water is
especially preferred. Generally, the drying fluid should be an
organic compound which is non-reactive with the surface to be dried
and has a boiling point less than 140 degrees Centrigrade at
atmospheric pressure.
The chemical found most effective for drying is isopropyl alcohol
(isopropanol). Isopropanol is economical, relatively safe
(nontoxic) and forms a minimum-boiling azeotrope with water. Also
of importance, isopropanol has a low surface tension and has both
hydrophobic and hydrophilic characteristics (i.e., it is miscible
in both oil and water). Without wishing to be bound by any
particular theory, it is believed that isopropanol has a tendency
to break the harsh surface tension between the hydrophilic water
and the relatively hydrophobic wafer surface. Since the solid phase
at interface 34 is the wafer surface and the liquid phase is ultra
pure water, the choice of gas phase properties can have a
tremendous impact which isopropanol appears to satisfy best.
The method of the present invention will now be described in more
detail with reference to the above described apparatus and the
preferred rinsing fluid (water) and drying vapor (isopropanol),
although it will be understood that the method can be carried out
with other suitable apparatus, and the method will be similar for
other rinsing and drying fluids.
In the wet processing of semiconductor wafers according to the
method of our U.S. Pat. No. 4,778,532, it is advantageous with some
process fluids to have the fluid flow upwardly through the vessel
12 so that the fluid inlet 24 becomes an outlet and the fluid
outlet 28 is the inlet. This is true of the rinsing fluid which
normally circulates upwardly through vessel 12. However, according
to the present invention, it has been found that the optimal
configuration for wafer drying is downflow.
Preferably, the last cycle of rinsing with ultra pure water is with
hot water (e.g., 65-85 degrees Centigrade) in order to heat the
wafers to approximately the boiling point of the isopropanol (82
degrees Centigrade). Alternatively, the wafers 16 may be heated by
direct solid solid heat transfer through wafer carriers 18, 20 by
means of heating bands or other heating devices applied to the
carriers.
After the final rinsing cycle, vessel 12 is left hydraulically full
with ultra pure hot water. Valve 78 may then be opened. However,
the water will not leave vessel 12 because nothing has been allowed
to enter at the top to replace it. Flash valve 62 and valve 70 are
then opened to introduced a stream of pure, saturated isopropanol
vapor to vessel 12 through inlet 24. As vapor enters the upper
vessel 22, water flows out the bottom 26 through fluid outlet 28
and valve 78.
Alternatively, the rinsing water may be removed from vessel 12 by
metering pump 79, with the flow rate being changed depending upon
the phase of the removal cycle. For example, the metering pump 79
can be run at a very high rate until the interface 34 is just above
the wafers, then slowed down as the interface descends past the
wafers. Finally, after the interface has passed the last wafer
surface 16, the bypass valve 78 around pump 79 may simply be opened
and the remaining water and drying fluid may be blown from the
vessel by purging gas.
It is believed to be important that the downward velocity of the
gas-liquid-solid interface 34 be controlled at a relatively slow
rate, although there is a compromise which must be taken into
consideration. Thus, if the rinsing water exits vessel 12 too fast,
liquid droplets will remain on the wafers and contamination will
occur when these droplets are evaporated. Hence, it is preferred
that the drying vapor displace the rinsing fluid from the wafers at
such a rate that substantially no liquid droplets are left on the
wafer surfaces after replacement of the water with isopropanol. On
the other hand, a faster descent of interface 34 increases dryer
productivity and minimizes chemical consumption.
Generally, interface descent rates (velocities) in the range of
about one to four inches per minute have been found satisfactory.
Descent rates much beyond five inches per minute yield poor
results, whereas descent rates below about one inch per minute are
inefficient. It has also been found that warmer vessel temperatures
of about 75 degrees Centigrade result in better drying performance
by allowing a faster interface descent than a cooler vessel at
about 60 degrees Centrigrade.
Similarly, consistent with the desire that liquids not be removed
from the wafer surfaces by evaporation, it is preferred that the
isopropanol be superheated in order to provide a drier vapor and
avoid the risk of condensation of a vapor on the wafer surfaces. An
advantage of an organic liquid such as isopropanol is that its
latent heat curve in its pressure-enthalpy diagram slopes
backwardly so that a pressure drop pushes the saturated vapor into
the superheated region of the phase diagram. As a result, passing
the saturated vapor produced by vaporizer 10 through a flash valve
62 results in a drier, superheated vapor being supplied to wafers
16 in vessel 12.
Holding section 38 of vaporizer 10 preferably holds enough liquid
IPA so that vaporizer 10 may supply several loads of wavers without
being replenished. When it is necessary to add fresh isopropanol to
vaporizer 10, this may be done automatically by allowing the
temperature of the boiling isopropanol in holding section 38 to
drop below its boiling point (82 degrees Centrigrade at atmospheric
pressure). The temperature lowering may be assisted by use of a
cooling jacket (not shown) on holding section 38. When the
temperature drops below the boiling point, the vapor pressure in
the vaporizer 10 becomes subatmospheric. When valve 54 is then
opened, liquid isopropanol is drawn from storage by suction to the
vaporizer. Because this liquid isopropanol is generally cooler than
the vaporizer itself, the pressure in the vaporizer reduces
further, enhancing the refill operation.
As rinsing water leaves vessel 12 through valve 78 or metering pump
79 to drain 81 or valve 83, limit switch 82 senses when vessel 12
has drained completely of liquid. As soon as the vessel is
completely empty, line 64 is closed and a stream of dry, inert,
non-condensable gas, such as nitrogen, is admitted through valves
71, 68 and 70 to purge vessel 12 of isopropanol vapor. Since the
nitrogen purge gas also goes through ceramic filter 69 between
valves 68 and 70, the filter is inherently purged with nitrogen
after the isopropanol. It is believed that this purge prevents
isopropanol condensation and resultant blinding problems inside the
filter.
This purge quickly flushes all of the remaining vapor out of vessel
12 through drain valve 78. The nitrogen purge displaces isopropanol
vapor out of the vessel and removes what appears to be a monolayer
of isopropanol on the wafer surfaces. The monolayer is very
volatile but the mechanism of removal appears different from
evaporation.
Finally, the last of the used water and the isopropanol from vessel
12 enter boiler 14. The purpose of boiler 14 and distribution
column 94 is to hold the isopropanol contaminated water mixture and
reconcentrate the isopropanol into a smaller volume for
recirculation or disposal. Once vessel 12 is emptied of liquid, the
nitrogen purge begins. This nitrogen gas flows through and out of
vessel 12 into boiler 14, exiting through condenser 86 and vent
88.
Since the isopropanol appears to be concentrated in a layer on top
of the water as interface 34 descends in vessel 12, it is not
necessary to strip all of the water which first exits vessel 12.
Therefore, most of the water is allowed to flow through valve 81,
and the last pint or so of liquid (i.e., the isopropanol layer and
water immediately below it) is trapped in boiler 14 for processing.
Since such a small amount of liquid is involved, and the
isopropanol/water 1mixture may be boiled readily.
As band heaters 92 are energized, the water/isopropanol mixture is
heated, and the azeotrope isopropanol water mixture (boiling point
79 degrees Centrigrade) vaporizes out of the waste water. This
gaseous blend goes through distillation column 94 to condenser 86
where it is condensed and is either collected in a waste-liquid
receiver 95 from drain 90 or is refluxed to distillation column 94.
Waste water from which the azeotrope and vapor have been removed
overflows through valve 96 before used water from the next run
enters boiler 14 from vessel 12.
Essentially, boiler 14 performs a crude separation, so that liquid
sent to distillation column 94 from condenser 86 might be 50 weight
percent wafer and 50 weight percent isopropanol. The distillation
column will then separate the azeotrope from the water to a 90
weight percent isopropanol/10 percent water azeotrope. This
azeotrope may be reintroduced to vaporizer 10 on a batch basis, or
continuously, as desired, through waste receiver 95 and appropriate
mixing tanks and/or filters.
As indicated previously, if any water is left on the water surface
after rinsing, streaking, spotting and particulate contamination
will almost invariably occur. An advantage of using a
minimum-boiling azeotrope, such as isopropanol with water, is that
any residual water film on the wafer surface, when combined with
the isopropanol vapor will immediately flash into the gas phase as
the azeotrope.
While one theory holds that the efficiency of drying depends upon
the thickness of the liquid isopropanol layer on the top of the
receding rinsing water at interface 34, a thin isopropanol liquid
layer or no layer at all is preferred due to fewer disposal
problems, lower drying fluid cost, and fewer flammability problems.
Regardless of the validity of this theory, it appears to be clear
that streaking and spotting are minimized by decreasing the
velocity of the receding water level (interface 34) and increasing
the dryness of the vapor (superheat).
While the above description contemplates either recycling of
isopropanol (closed loop system) or merely the stripping of the
mixture to concentrate the organic liquid for disposal, purifying
the isopropanol or other drying vapor for recovery and recycling is
preferred because of environmental and economic considerations. The
azeotrope mixture has a lower flash point and better heat transfer
characteristics including heat capacity and heat transfer
coefficient, apparently without sacrifice of particle performance
(cleanness) of the wafers compared to the use of fresh, pure
isopropanol alone.
Without wishing to be bound by any particular theory, it is
believed that if interface 34 recedes at a sufficiently slow rate,
the water/isopropanol interface will pull all of the water and
azeotrope droplets off the wafers as the liquid level recedes.
However, if the interface 34 recedes too quickly, the surface
tension or affinity of water or other droplets for the wafer
surface will exceed the surface tension of the receding liquid.
Drops will then be left on the wafer which will have to be
evaporated, leaving streaks and contamination. Thus, it is believed
that the method of the invention involves a physical pushing by the
vapor or pulling by the liquid surface, resulting in direct
displacement by vapor for liquid rather than an evaporation of
liquid droplets.
It will be appreciated from the above description that the method
and apparatus of the invention may be an enclosed, full flow system
which is preferably hydraulically full (i.e. sealed) during both
rinsing and drying steps, and requires no movement or handling of
the wafers between rinsing and drying. The advantages of such
systems are disclosed in our U.S. Pat. No. 4,778,532.
The present invention results in improved drying efficiency of
semiconductor wafers with less contamination of the wafer surfaces
by dissolved particles and other contaminants which may be present
in prior systems.
The present invention may be embodied in other specific forms
without departing from the spirit or essential attributes thereof,
and, accordingly reference should be made to the appended claims,
rather than the specification, as indicating the scope of the
invention.
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